First Law for Open Systems

Beginning with the general balance equation, we can develop an expression for the open system energy balance. For now, lets work with a SISO (single input, single output) system. We can always add the extra terms for extra inlet and outlet streams if needed.

energy balance
Each element of fluid possesses internal, kinetic, and potential energy, so:
specific energy

Next, look at the work term. Recall that there are three types of work:

Just as we did for the open system mass balance, we usually do our best to set up the system with fixed volume; therefore, there will not be any boundary movement work, and the work term can be written:
work
Notice how the sign convention applies -- work is done on the system (negative) to move fluid into the system, work is done by the system (positive) to move flow out of the system.

When everything is assembled, we apply the definition of enthalpy to get:

rules
This is equivalent to equation 6.7 in the text and is important. This is a very general form of the open system energy balance, and will be the starting point for developments in the next several classes.

References:

  1. Cengel, Y.A. and M.A. Boles, Thermodynamics: An Engineering Approach (3rd ed.), WCB McGraw-Hill, 1998, p. 197-200.
  2. Sonntag, R.E., C. Borgnakke, and G.J. Van Wylen, Fundamentals of Thermodynamics (6th ed.), John Wiley, 2003, p. 165-67.
  3. Sonntag, R.E., C. Borgnakke, and G.J. Van Wylen, Fundamentals of Thermodynamics (5th ed.), John Wiley, 1998, p. 143-45.

R.M. Price
Original: 5/1/2000
Modified: 5/25/2000; 10/18/2003

Copyright 2004 by R.M. Price -- All Rights Reserved

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